Yes over estimate.
I did read that the porous media doesn't occupy a volume. So to account for the solid volume I added a ternary phase with a constant velocity of 0 m/s in all zones. I then patched the volume fraction of the solid phase equal to (1-porosity) = 0.03. This enforces the volume fraction limitation of 0.970 (V_L+V_S+V_G=1).
What I am regarding is that in their results their average volume fraction of the liquid phase in the porous zone is ~0.40 (see image way up above).
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